Wrap the sweet potatoes in baking paper and aluminum foil. Bake in an oven preheated to 355°F (180°C) for about an hour until they soften.

Take the sweet potatoes out of the oven, place them on a plate, cut an oval shape from the top, and remove a little of the sweet potato. Mash the removed portion with a fork.

In a bowl, mix egg yolks, sugar, and cornstarch until smooth. Add the heavy cream and mix well. Transfer to a small pot and cook over medium heat, stirring until it thickens. (The Japanese prefer it quite liquid, but you can cook it longer if desired; just be careful not to overcook.)

Immediately fill the sweet potatoes with the cream and let them cool slightly.

(Optional: You can put them back in the oven for half an hour, similar to crème brulee. For a cold version, refrigerate.)

Sprinkle sugar on top and caramelize with a kitchen torch.